Publish/subscribe data processing systems have become very popular in recent years as a way of distributing data messages from publishing computers to subscribing computers. The increasing popularity of the Internet, which has connected a wide variety of computers all over the world, has helped to make such publish/subscribe systems even more popular. Using the Internet, a World Wide Web browser application (the term "application" or "process" refers to a software program, or portion thereof, running on a computer) can be used in conjunction with the publisher or subscriber in order to graphically display messages. Such systems are especially useful where data supplied by a publisher is constantly changing and a large number of subscribers needs to be quickly updated with the latest data. Perhaps the best example of where this is useful is in the distribution of stock market data.
In such systems, publisher applications of data messages do not need to know the identity or location of the subscriber applications which will receive the messages. The publishers need only connect to a publish/subscribe distribution agent process, which is included in a group of such processes making up a broker system (referred to hereafter as a broker), and send messages to the distribution agent process, specifying the subject of the message to the distribution agent process. The distribution agent process then distributes the published messages to subscriber applications which have previously indicated to the broker that they would like to receive data messages on particular subjects. Thus, the subscribers also do not need to know the identity or location of the publishers. The subscribers need only connect to a distribution agent process.
One such publish/subscribe system which is currently in use is shown in FIG. 1. Publishers 11 and 12 connect to the publish/subscribe broker 2 and send published messages to broker 2 which distributes the messages to subscribers 31, 32, 33, 34. Publishers 11 and 12, which are data processing applications which output data messages, connect to broker 2 using the well known inter-application data connection protocol known as remote procedure call (or RPC). Each publisher application could be running on a separate machine, alternatively, a single machine could be running a plurality of publisher applications. The broker 2 is made up of a plurality of distribution agents (21 through 27) which are connected in a hierarchial fashion which will be described below as a "tree structure". These distribution agents, each of which could be running on a separate machine, are data processing applications which distribute data messages through the broker 2 from publishers to subscribers. Subscriber applications 31, 32, 33 and 34 connect to the broker 2 via RPC in order to receive published messages.
Publishers 11 and 12 first connect via RPC directly to a root distribution agent 21 which in turn connects via RPC to second level distribution agents 22 and 23 which in turn connect via RPC to third level distribution agents 24, 25, 26 and 27 (also known as "leaf distribution agents" since they are the final distribution agents in the tree structure). Each distribution agent could be running on its own machine, or alternatively, groups of distribution agents could be running on the same machine. The leaf distribution agents connect via RPC to subscriber applications 31 through 34, each of which could be running on its own machine.
In order to allow the broker 2 to determine which published messages should be sent to which subscribers, publishers provide the root distribution agent 21 with the name of a distribution stream for each published message. A distribution stream (called hereinafter a "stream") is an ordered sequence of messages having a name (e.g., "stock" for a stream of stock market quotes) to distinguish the stream from other streams. Likewise, subscribers provide the leaf distribution agents 31 through 34 with the name of the streams to which they would like to subscribe. In this way, the broker 2 keeps track of which subscribers are interested in which streams so that when publishers publish messages to such streams, the messages can be distributed to the corresponding subscribers. Subscribers are also allowed to provide filter expressions to the broker in order to limit the messages which will be received on a particular stream (e.g., a subscriber 31 interested in only IBM stock quotes could subscribe to the stream "stock" by making an RPC call to leaf distribution agent 24 and include a filter expression stating that only messages on the "stock" stream relating to IBM stock should be sent to subscriber 31).
Oftentimes, it is necessary for a distribution agent to inform its "children" (i.e., the distribution agents directly underneath the distribution agent) of some event, so that the children can take some appropriate action. For example, if distribution agent 22 should lose its connection to its parent (the root distribution agent 21), distribution agent 22 needs to inform its children distribution agents 24 and 25 so that such children can take an appropriate action which has been pre-configured by the systems administrator (e.g., the distribution agents 24 and 25 could either try to connect to another parent, such as distribution agent 23, or they could simply stay with distribution agent 22 and wait until distribution agent 22 regains its connection with the root distribution agent 21).
In the prior art publish/subscribe broker, this type of communication between a parent distribution agent and its children has involved the parent having to send a dedicated command, outside of the normal publish/subscribe message flow. This increases the types of traffic flowing between distribution agents. Further, one child may receive messages in a different order as compared to one of its siblings, due to the fact that the parent must send separate commands to each child. Accordingly, the prior state of the art in this area has resulted in an inefficient use of available resources and a generally unsatisfactory architectural approach.